1. Field of the Invention
The present invention relates to an ESD protection device, and more particularly to an ESD protection device causing discharge to generate between opposing portions of discharge electrodes, which are exposed to a cavity formed inside a ceramic multilayer substrate.
2. Description of the Related Art
ESD (Electro-Static Discharge) implies a phenomenon that strong discharge occurs when a conductive object (e.g., a human body) charged with electricity contacts with or comes sufficiently close to another conductive object (e.g., an electronic device). The ESD gives rise to a problem such as damage or malfunction of the electronic device. To avoid the problem, an excessive voltage generating in the event of discharge needs to be kept from being applied to circuits of the electronic device. An ESD protection device is used in such a case, and it is also called a surge absorption device or a surge absorber.
The ESD protection device is arranged, for example, between a signal line of a circuit and a ground (earth). The ESD protection device has a structure that a pair of discharge electrodes face each other with a spacing held therebetween. In a state of ordinary use, therefore, the ESD protection device has a high resistance, and a signal does not flow to the ground side. However, if an excessive voltage is applied such as when static electricity is applied from an antenna of a cellular phone, for example, discharge occurs between the discharge electrodes of the ESD protection device, whereby the static electricity can be conducted to the ground. As a result, a voltage due to the static electricity is not applied to circuits downstream of the ESD protection device, and those circuits can be protected.
In an ESD protection device 1 illustrated in a sectional view of FIG. 10, for example, a cavity 3 and discharge electrodes 6 and 8 facing each other with a spacing 5 held therebetween are formed inside a ceramic multilayer substrate 2. The discharge electrodes 6 and 8 include opposing portions 7 and 9 formed along an inner surface of the cavity 3. The discharge electrodes 6 and 8 extend from the cavity 3 to an outer peripheral surface of the ceramic multilayer substrate 2, and they are connected respectively to outer electrodes 6x and 8x, which are formed outside the ceramic multilayer substrate 2, i.e., on the surface of the ceramic multilayer substrate 2. The outer electrodes 6x and 8x are used for mounting of the ESD protection device 1. An auxiliary electrode 4 in which a conductive material is distributed is formed adjacent to the opposing portions 7 and 9 of the discharge electrodes 6 and 8 and to a region where the spacing 5 between the opposing portions 7 and 9 is formed.
When a voltage higher than a predetermined value is applied to the outer electrodes 6x and 8x, discharge occurs between the opposing portions 7 and 9 of the discharge electrodes 6 and 8. With the discharge, the excessive voltage is conducted to the ground, whereby circuits on the downstream side can be protected. A discharge starting voltage can be set by adjusting the spacing 5 between the opposing portions 7 and 9 of the discharge electrodes 6 and 8, the amount and type of the conductive material contained in the auxiliary electrode 4, etc. (see, e.g., Patent Document 1).
Patent Document 1: International Publication No. 2008/146514